THX Goes Mobile

I really needed a vacation last month. Work was hectic as usual, and I had a few personal problems that piled on top of me all at once, so I felt a strong desire to get out of Dodge (well, Burbank, actually). I was about to hop in my car and hit the road when THX came to my rescue and offered me a far more stylin' ride: a 2006 Lincoln Zephyr with a THX-certified, CD-based audio system they wanted me to review.

There was only one problem: the car was in San Francisco. They would let me drive it back to LA, but first I had to get there to pick up the car. I could have flown, of course, but when I get stressed, I like to drive the open road, so I rented a car. (Not many companies do one-way rentals; thanks, Alamo!) I got a Chevrolet Malibu Maxx, grabbed some CDs, and headed up Interstate 5.

As soon as I hit "play" on the Chevy's sound system, I realized that I was probably in for a real treat—on the way back. The sound in the rental car was fuzzy, indistinct, and bloated, even after I readjusted the tone controls from the maxxed-out positions the last driver had left them in. Surely, I thought, a THX-certified system would blow this piece of crap out of the water.

And so it did. After a lovely lunch and comprehensive orientation with Laurie Fincham, THX's extremely knowledgeable chief scientific officer (and a bass player, to boot!), he handed me the keys to one of only four 2006 Zephyrs in the country and bid me bon voyage.

Sitting Still
The $30,000 Zephyr is a new model from Lincoln designed to attract a somewhat younger buyer than their other cars, for which the average owner age is somewhere north of 60. (Actually, it's the latest incarnation of that model name, which began in the 1930s as an upscale sporty car.) The overall styling is clean but fairly nondescript. According to Fincham, the exterior is smaller than the Lincoln LS, but the interior is actually larger. The ride is ultra-smooth and quiet, and the V6 engine has plenty of pep.

Lincoln approached THX four years ago to help them design the best possible automotive sound system. They started by evaluating the stock system in a Lincoln LS, which they then improved in the garage with external rack-mounted components. Of course, it would be rather difficult to drive down the road with a rack trailing behind, so the next step was to make it mobile. Phase I of the project consisted of tweaking the stock systems in the LS, Navigator, Aviator, and Town Car models.

In Phase II, THX started more from scratch with the Zephyr, though they were still limited by the space available in the doors, dashboard, and rear deck. Their goal was a lofty one: to replicate a THX-certified home system's performance as much as possible in a car. To achieve this goal, THX did most of the R&D work, designing the speaker drivers, amps, crossovers, and DSP functions. The result is a factory option that adds $1000 to the price of the car.

Factory amplifiers are typically underpowered, and cars are naturally noisy environments, so the undistorted peak level had to be increased substantially. THX decided to go with so-called class H amplifiers, which can produce high power while remaining relatively small and lightweight. In the Zephyr, they provide 12 channels of amplification with a total power of 660W (55Wpc into 4Ω). Interestingly, the automotive industry typically specifies maximum audio power at 10% distortion, but the THX spec is measured at the point where clipping just starts to occur, which corresponds to about 1% distortion.

The amps are "smart" in that they use digital signal processing to modify their behavior depending on the program material and volume setting. This ensures that the amps don't get too hot and that the speakers aren't driven too hard, especially since THX did not want to overdesign the speakers, which would impede their performance. The DSP, which resides in the amps rather than the head unit, monitors the peak and average levels and adjusts the master gain if any channel is going into distress. It also implements a form of soft clipping to further reduce distortion. None of this occurs except at very loud volumes; THX claims the system can produce a maximum peak level of 125dB SPL (unweighted) or 113 dB (C weighted).

THX also spent a lot of time and effort on the speaker drivers, of which there are 14: a 5x7" woofer and 1" dome tweeter in each front door, two 2x3" midranges and one 1" dome tweeter in the center of the dashboard (a design dubbed CSA, for Configurable Speaker Array), a 5x7" woofer/tweeter coaxial pair in each rear door, and a 4" mid/high driver and two 6x9" subwoofers in the rear deck. Each driver is powered by its own amp, except the 2-way rear-door coaxial speakers, each of which is driven by a single amp. There wasn't much room for the CSA in the dashboard, so THX developed a slotted speaker aperture that's only 3/8" wide.

The CSA makes ingenious use of a small space in the dashboard.

With so many drivers, it's important to keep in mind that this is a CD-based system, not multichannel DVD-Audio or SACD. The whole idea was approved three years ago, when there wasn't much software in these formats—which is still more or less true today. But the system is designed to accommodate multichannel playback in the future. It also includes an AM/FM radio, but no satellite radio, which is rather surprising with such an upscale system.

Then there's the issue of balance; in a car, the balance of channels is radically different at each seating position. THX made good use of the DSP to implement a modified version of their Advance Speaker Array (ASA) algorithm found in Ultra2-certified AV receivers. In that case, the algorithm uses the difference signal from the two surround channels to affect the side and rear surrounds, but in the car-audio system, it is applied to the front LCR drivers to widen the sweet spot and spread the image. In fact, the L/R signal from a CD undergoes a complex upmix to 12 channels of amplification, with different equalization and ASA processing for each channel and low-frequency redirection to the subs.

All of this processing is applied in three preset, user-selectable modes: Driver, Front, and All Seats. As the names imply, these modes optimize the balance for the driver's seat, both front seats, and all seats, respectively. Also available are Fade (front-to-back) and Balance (right-to-left) controls, but these don't pull the signal all the way to the extremes, which would defeat the purpose of a THX-certified system.

The only tone controls are Bass and Treble, which provide ±6dB shelving adjustments. THX felt that a graphic EQ would also defeat the purpose of their optimization, but I would prefer to have the option.

According to THX, the free-field frequency response of the system is 30Hz (-6dB) to 18kHz (-3dB), with 20Hz down by -12dB. In the car, things are somewhat different, with an intentional emphasis around 30Hz that rolls off below and drops slightly to a plateau above until 5kHz or so, where it gently rolls off into the upper frequencies. This is typical of the measured in-car response of a well-balanced system and is close to the power response within a small, reflective volume. THX did everything they could to minimize the effect of the inevitable resonant modes in such an unforgiving environment.

The perceived dynamic range of the system while the car is in motion is said to be around 60dB, which isn't much compared with a good home system. But this isn't a home system; the noise floor in a car is generally high (typically 85-90dB SPL when moving down the road), so an extended dynamic range is not useful in this case. In addition, the CD player has a compression circuit that reduces the dynamic range by 20dB when engaged. (The radio has no compressor, since radio signals are usually compressed before transmission.)

Finally, a control called Speed Compensated Volume (SCV) raises the volume as the car's speed increases, and it can be set to one of seven levels of sensitivity. This is based on the theory that there is more noise at higher speeds, which isn't necessarily true; it's entirely possible to have more noise driving slowly over a rough road than driving fast on a smooth highway. I left it off most of the time.

On the Road
Once I understood the system's goals and features, it was time to hit the road. I brought a number of CDs representing a variety of musical styles in order to give the system a thorough workout. I started with Steely Dan's Two Against Nature (Warner Bros. 9 24719-2), a superbly recorded disc with plenty of energy in all frequency ranges. My first impression was—impressive! The sound was very clean with plenty of volume, and the electric bass was quite punchy. However, the bass range was also a bit overbearing and boomy. Turning down the Bass control a couple of dB seemed to even it out.

The same was true with 2 Many Axes (pfMENTUM CD 020), the second CD by my avant-garde acoustic trio, Many Axes, which I mixed and helped master. The opening track includes a suling (Indonesian flute), ocarina, and a very deep bass drum, all played quite softly. With the Bass control at 0, the drum was overpowering, but once I turned it down a couple of dB, it sounded fine, with all the depth we intended to balance the higher flute sounds. This disc also showcases the treble end of the spectrum with tinkly wind chimes and high-pitched whistles, which the THX system reproduced with aplomb.

Turning to orchestral music, I played Shostakovich's Symphony No. 8 as recorded by the Dallas Symphony Orchestra under Andrew Litton (Delos DE 3204). Like the Many Axes CD, this one has a very wide dynamic range, and it sometimes fell below the noise floor, so I engaged the compressor, which helped a lot. Otherwise, the power of this well-recorded CD came through loud and clear, with each instrumental section unmistakably delineated.

One of my favorite test CDs is the eponymously titled Take 6 (Reprise 9 25670-2), a group of six male vocalists singing a capella with intonation so perfect it almost hurts. The group includes a deep bass, at least one guy who can sing well into the female registers, and everything in between. The blend was exquisite, and intelligibility was no problem at all.

On my way home, I stopped overnight at an audiophile pal's house, and we listened to the system together using some of his electronica CDs, including Kraftwerk's Expo 2000 Remixes (EMI 7243 8 87984 2 6, German import) and Yello's Eye (Universal 06024 9808552 3). His immediate reaction was that the bass was wimpy, especially in the lowest octave. I had the Bass control turned down, so we reset it to the default position, which helped improve his opinion. (I still thought it was too much.) On the other hand, he liked the bass punch and the rendering of female vocals on Eye. In an admittedly unfair comparison, we played some of the same tracks on his $30,000 home system (MartinLogan Prodigy speakers, Classé CAM 350 monoblock amps, Lexicon MC-12 preamp, Denon 2910 disc player, KimberKables), which did have more bass extension (duh!), but not so much more that I felt deprived in the car.

We decided that the Zephyr's midbass was probably boosted to compensate for a lack of extreme low bass, which turns out to be exactly the case (though we didn't know it at the time). After all, how much 20Hz energy can one expect from two 6x9" subwoofers powered by 55W each? In the context of a car audio system, I have no problem at all with the Zephyr's bass extension.

Regarding the three DSP balance modes, I ended up preferring the All Seats mode, which spread the sound more evenly around me. I really enjoy surround music on DVD-Audio and SACD, and All Seats mode simulated that experience better than the other two modes. And because this mode lowers the subwoofer level slightly to prevent rear passengers from being blasted out of their seats, it helped tame the low end without requiring more drastic cuts in the Bass control.

To test the noise floor and playback levels, I brought along a Larson Davis 712 SPL meter. With no audio playing and the air-conditioning fan at minimum, I measured an Leq (average level over a given time interval) of 62dB (A weighted, five minutes) on surface streets and 71dBA on the highway. To see how loud the system would go, I played "Synchronicity" from the Police album of the same name (A&M CD-3735) at full volume. With both the compressor and SCV off, I measured an Leq of 91dBA on the highway; with the compressor on, it rose to 94dBA, and with SCV on, it topped out at 92dBA. I have no doubt that C-weighted peaks of 113dB were reached, though I did not measure them specifically. In all cases, there was little if any perceptible distortion, indicating that the soft clipping and limiting work very well indeed.

The only real complaint I have is with the user interface, which is an LCD touchscreen with some "hard" buttons flanking it. For one thing, in certain lighting conditions, fingerprints on the LCD are obvious, but I'm not sure if there's anything they could do about that. Also, the audio-control display is monochrome, even though the navigation-system display is full-color. More frustrating is the fact that CD playback stops when you swap out one of the other CDs in the slot-loading 6-disc changer. Unless you're ejecting the currently playing CD, it should keep playing, as many other changers do. Finally, the audio controls should time out and be replaced by a large version of the disc/track/time info in the main display.

The LCD-touchscreen user interface is the only part of the system I didn't particularly like.

But these issues don't affect the experience of listening to music in the car, which is exceedingly enjoyable. Of course, it is completely unreasonable to compare the $1000 THX car-audio system to a $30,000 audiophile home system (which cost about the same as the entire car!), but I have no problem comparing it to the sound in the rental car I drove on the outbound leg of my journey—and, in fact, there's no comparison. The THX system blew it away with clean, clear, powerful sound that's far more musical than one normally expects in an automotive system. If you're in the market for an upscale ride with upscale sound, the 2006 Lincoln Zephyr with the THX option should definitely be on your short list.

Now, sadly, I must return the car. I think I'll take the long route to the THX offices in Burbank—by way of San Diego!